1. Field of the Invention
The present invention pertains to flow control devices. More specifically it pertains to flow control devices suitable for controlling a well during drilling operations. More specifically it pertains to such flow control devices which are known in the industry as "blowout preventers".
2. Description of the Prior Art
Since the early days of the petroleum industry, "blowout" of a well during drilling operations has been a major concern. If the proper precautions are not taken, the drill bit may enter a high pressure formation causing oil and/or gas to rush out of the well creating hazards to both life and property.
Although use of the first mechanical well control equipment was apparently not recorded, the development of numerous inventions to prevent blowouts has occurred since the late nineteenth century up to the present date. Continuing developments bear witness to the unending search for new and improved method of preventing loss of control of well pressures at the surface.
The flow control devices, known in the industry as blowout preventers, for controlling well pressures may be classified under one of three broad forms: the inverted packer type, the ram type and the stuffing box or pressure operated drilling packer type. The purpose of any of these types of blowout preventers is to seal the annular space between the drill stem and the casing quickly, easily and safely.
One of the most popular and widely used of these three types of blowout preventers is the ram type. Such preventers generally comprise a housing which may be attached to the well casing and which is provided with a passageway through which the drill string may be passed. Carried in the housing is a pair of rams which are disposed for reciprocal movement between retracted positions, in which the annular area between the drill string and the blowout preventer passageway is open, and an extended position, in which the rams engage the exterior of the drill string and sealingly close the annular space between the drill string stem and casing. The rams are usually connected by a rod to a piston and cylinder assembly carried by the blowout preventer housing. To close the blowout preventer (to move the rams to the extended or sealing position), pressure is applied to the piston and cylinder assembly forcing the rams into sealing engagement with the drill string. To open the blowout preventers, pressure is simply applied to the opposite end of the piston and cylinder assembly. The most popular ram blowout preventers are made by Cameron Iron Works, Inc. and Schaffer Tool Works, a subsidiary of the Rucker Company. Blowout preventers made by these companies may be seen in the 1972-73 revision of the Composite Catalog of Oilfield Equipment and Services published by the Gulf Publishing Company.
When a blowout preventer is closed in a well that is "blowing out" or threatening to "blowout" or when the well is to be left unattended, it is desirable to lock the rams in this closed position. Otherwise, extreme pressures must be maintained on the piston and cylinder assemblies of the blowout preventer. In the past, a manually operated locking screw has been used which, when screwed in place, engages a "tail rod" attached to the piston of the blowout preventer piston and cylinder assembly, preventing return of the piston to the retracted or open position. Such locking apparatus utilized in one of Cameron's blowout preventers may be seen at page 960 and 961 of the aforementioned Composite Catalog.
Although a relatively long period of time may be required to engage such locking apparatus, it has been found to be suitable for most land based drilling operations. However, such locking apparatus may not be suitable for subsea drilling. Much subsea drilling is conducted with the blowout preventers placed near the floor of the body of water in which the drilling is being conducted. Operating a manually operated locking mechanism at the bottom of an ocean is impractical if not impossible. Therefore, other locking apparatus has been developed.
One such remotely operable locking apparatus, developed by Cameron, is shown on page 962 of the Composite Catalog. In such a blowout preventer, the tail rod, which is attached to the operating piston, is tapered on the end for engagement by a hydraulically actuated wedge member which is mounted for reciprocal movement along a path generally perpendicular to the axis of the tail rod. When it is desired to lock the blowout preventer in the closed position, the rams are closed and pressure held thereon until pressure is applied to the wedge member. The wedge member is wedged behind the end of the tapered tail piece. One advantage of such locking apparatus is its adjustability, allowing the ram to be locked wherever it stops regardless of wear to the ram seal, etc. Another advantage is that the lock is not actuated every time the ram closes but only when it is desired to lock the rams. However, there are some disadvantages of such locking mechanisms. Occasionally the tapered surfaces are wedged so tightly together that they cannot be released by hydraulic pressure. Furthermore, if the planes on the tapered wedge or the tapered end of the tail rod have any lubricant on them, it is possible to apply enough force to the rams to release the locking device. In addition, such a blowout preventer requires at least four hydraulic hoses. The more hydraulic hoses required in a subsea installation, the greater the connection and maintenance problems.
Another type of locking apparatus, which is used by Schaffer Tool Works and which may be seen on page 3873 of the aforementioned Composite Catalog, is the type in which the preventer piston assembly is provided with radial latches which, upon closing, automatically engage annular surfaces within the cylinder to positively lock the rams in the closed position. One advantage of such lockout apparatus is that only two hydraulic hoses are required for the blowout preventer. However, there are also some disadvantages of this type of lock. For one, the lock is actuated each time the blowout preventer is closed because the lock is operated by closing pressure. This is a disadvantage in that it is not always desired to lock the rams each time they are closed but only on occasions when the well is actually blowing out, threatening to blow out or is to be left unattended (such as when the rams are closed to hang the drill string and the drilling vessel is moved due to bad weather or any other reason). Furthermore, actuating the locks each time the rams are closed may cause excessive wear and a greater chance of malfunction when the lock is actually needed. In addition, this type of lock locks the ram in exactly the same position every time. This is a disadvantage because the rams may need to be locked in a further closed position as the ram seals wear. Otherwise, the ram may be locked before it has traveled inwardly enough to completely seal off against the drill string. Furthermore, most blowout preventer rams are made to run over the center and when one ram has moved past the locking position the other ram may not have moved enough to be locked.
A still further disadvantages of either of the above mentioned locking systems is that there is no good way to check whether or not the lock has been effected. The radial latch type lock cannot be checked since applying ram opening pressure would unlatch the lock. Opening pressure can be applied to the wedge type lock to determine whether or not it has been effected, but a low opening pressure will not assure that it is locked since the ram might be slightly hung or stuck and not actually locked. Therefore a high pressure must be applied to be sure the lock is effected and this tends to overload the locking device.
Great strides have been made in the development and improvement of blowout preventers. Improvements have also been made in locking such blowout preventers in the closed position. However, it is apparent that the present state of the art in locking blowout preventers still leaves much to be desired in efficiency, reliability and other operating, manufacturing and maintenance characteristics.